The present invention relates in general to a process for depositing a dielectric layer such as Ta2O5 on a substrate capable of being nitrided.
The increase in the integration density and the speed of electronic circuits is leading to a regular reduction in the size of the components such as elementary transistors. Because of this, the advance in fabrication technology is in particular characterized by a decrease in the thickness of some of the materials which are used, in particular the gate dielectric material for the transistors, permitting greater integration.
For this reason, it has been envisaged to replace the dielectric material conventionally used, SiO2, by materials with better performance. One of the materials adopted is tantalum pentoxide Ta2O5, because it has a higher electric permitivity than silicon oxide.
Ta2O5 is conventionally deposited by chemical vapour deposition (CVD) or plasma enhanced chemical vapour deposition (PECVD). The quality of the deposit depends on the surface condition of the substrate and, in particular, its surface energy which should, as far as possible, be predetermined, constant and reproducible. Furthermore, it is necessary to carry out a heat treatment of the deposited layer in order to repair the defects induced during fabrication. These treatments, generally oxidizing treatments, may greatly reduce the electric permitivity of the deposited layer owing to the diffusion of the species through the dielectric.
The process herein described relates to a process for depositing a Ta2O5 dielectric layer which overcomes the drawbacks of known processes.
In one embodiment, a process has been developed for depositing a Ta2O5 dielectric layer on a substrate capable of being nitrided, which includes the following steps:
pretreatment of a surface of the substrate by means of a cold gas plasma at low or medium pressure in order to clean the said surface;
growth, from the said cleaned surface of the substrate, of a nitride barrier layer by means of a cold gas plasma made up of an N2/H2 mixture at low or medium pressure; and
deposition, on the nitride barrier layer, of a Ta2O5 dielectric layer by chemical vapour deposition (CVD) or plasma enhanced chemical vapour deposition (PECVD).
The term cold plasma used in this patent application denotes non-equilibrium plasmas which have little energy transfer by atom/electron collisions and therefore include cold atoms (or radicals) as opposed to hot plasmas (stellar plasmas or thermonuclear fusion plasmas). These low-collision plasmas whose electronic temperature is of the order of a few electron volts are furthermore out of local thermodynamic equilibrium. The plasmas used for various treatments in microelectronics (etching, deposition, cleaning, etc.), and even more generally for the surface treatment of materials, are always of the xe2x80x9ccold plasmaxe2x80x9d type as defined above.
The expression ne greater than 1011cmxe2x88x923 refers only to the electron density. Nevertheless, the parameter ne is quite clearly decisive for the creation of active species in the plasma. The criterion ne greater than 1011cmxe2x88x923 therefore corresponds to a very high-density plasma leading to the production of a large concentration of these species, allowing effective surface treatment in a very short time. However, the value of ne is not in fact very critical, and it is perfectly possible to carry out treatments of this type with plasmas having a lower electron density, ne less than  less than 1011cmxe2x88x923, if it is acceptable to use processes which last longer (for example collective sample treatment in a batch reactor).
The operating pressure range for plasma treatment processes is directly linked with the type of plasma reactor (source) which is used. Several types of reactors which operate with different pressure intervals may be suitable for this application. These pressure intervals do not differ significantly from those of processes (deposition, etching) for which the reactors are usually designed. An example of a reactor which operates at xe2x80x9cmedium pressurexe2x80x9d (50-1000 mtorr) is given by the RF diode type (PECVD oxide deposition). The most recent generation plasma sources (ECR, DECR, inductive coupling helicon, etc.), used both for deposition and for etching constitute examples of systems which operate at xe2x80x9clow pressurexe2x80x9d (typically 0.5-10 mtorr).
The substrates to which the process applies are any substrates which are composed of a material capable of being nitrided, that is to say able to form a nitride, such as semi-conductor materials like Si, SiO2 and GexSi1-x(0 less than xxe2x89xa61), conductive materials like metals, for example Ti, W, Al and Cu, or alternatively polymers having polar groups.
The preferred materials for the substrates which can be used in the process are semi-conductor materials and, in particular, Si, SiO2 and GexSi1-x.
The first step in the process is a step of pretreating a surface of the substrate by means of a cold gas plasma at low or medium pressure in order to clean the surface of its impurities, for example the native oxide in the case of a silicon substrate, without inducing significant surface irregularities. The gas plasmas preferably used are plasmas made up of hydrogen, mixtures of hydrogen and argon, and oxygen. The particularly recommended plasmas are hydrogen plasmas. Oxygen plasmas are used for cleaning substrates which have a hydrocarbon surface or a surface which is highly contaminated, for example with oils.
Pretreatments for cleaning the surfaces of substrates using a plasma are known per se.
Preferably, this pretreatment step is carried out at a temperature below 300xc2x0 C., and even better at room temperature. The pressure is generally between 10xe2x88x924 and 1 torr, preferably between 10xe2x88x923 and 10xe2x88x922 torr.
The plasma density ne is 1010cmxe2x88x923 or more, and preferably ≳1011cmxe2x88x923, in order to shorten the pretreatment time.
This pretreatment step generally lasts between 1 second and 1 minute, and is preferably of the order of 30 seconds, in order to avoid the formation of significant roughness in the surface.
This plasma pretreatment should provide a low ion bombardment energy so as not to create defects in the surface which is treated. This is why the pretreatment is preferably carried out with a difference between the potential of the plasma and the floating potential, Vp-Vf less than 20 V.
The second step of the process includes a step of growing a nitride barrier layer from the pretreated surface of the substrate, in which step the material of the substrate contributes to the formation of the nitride barrier layer.
This growth step is carried out by using a cold plasma at low or medium pressure, made up of an N2/H2 mixture, preferably a high-density plasma. This step of growing the nitride barrier layer is generally carried out at a temperature below 300xc2x0 C., preferably at room temperature. The nitrogen and hydrogen delivery rates are generally in an N2/H2 ratio of about 1.5/1 to 4/1, preferably of the order of 2/1.
This growth step generally lasts between 5 and 15 minutes, preferably 7 to 10 minutes.
Thus, in the case of a silicon substrate, a barrier layer made up of silicon nitride Si3N4 is formed in this growth step.
The growth of this silicon nitride layer naturally saturates at a thickness of 3 nm (measured by ellipsometry, index n fixed at 3) by stopping diffusion of the reactive species through the layer. This is due essentially to the fact that use is made of plasmas with low ion bombardment energy (Vp-Vf less than 20 V) and with little thermal assistance.
The nitride barrier layer, which modifies the surface bonds of the substrate, leads to an increase in the polar part of the surface energy. This nitride barrier layer has a stabilized, uniform and reproducible surface energy, which guarantees control over the subsequent deposition of Ta2O5 and improves the adhesion of the Ta2O5 deposit. Lastly, this nitride barrier layer prevents oxidation of the underlying layers during subsequent treatments, such as annealing operations, for example after the Ta2O5 layer has been deposited.
In the case of an Si3N4 barrier layer, this layer was characterized by measuring its thickness using ellipsometry.
The non-polar and polar parts of the surface energy were also measured using the method consisting in measuring the contact angle of a liquid drop with the surface of the barrier layer. By measuring the contact angle of a drop of diiodomethane, the non-polar component of the surface energy is determined, the value of which is directly linked with the chemical nature of the surface. Measuring the contact angle of a drop of water characterizes the polar component of the surface energy.
One commercially available instrument for measuring the contact angles is the G2/G40 instrument from the company KRxc3x9cSS.